The present invention relates to a method for cleaning the surface of artwork painted surface lacquered, patina surface especially to a cleaning process using a laser of predetermined wavelength to interact with an agent in a contaminant surface on the artwork.
Laser technology is used in the field of painting conservation, to remove varnish films, polymerized overpaint, or calcium carbonate gesso layers (i.e. metallic or organic bonds) which cover and obscure oil paint films on canvas, wood, terracotta, paper, cardboard, metal, leather, parchment, plaster, and marble supports. The traditional method of cleaning painting surfaces is to dissolve or soften natural resin varnish films, overpaint or polymerized synthetic resin varnish, using aromatic or alkaline solvent. The solvent dissolves the substance or loosens its adhesive bond by penetrating the structure and breaking its molecular bonding. Normally a natural resin varnish or accumulation of soot and grease is dissolved with a mild solvent with little or no penetration into the paint layer, because the dissolved film is immediately neutralized as it turns to solute. When, however, a color surface is covered by a cross-linked or partially cross-linked varnish or paint layer, the solvent penetration requires a substantial rate of attack which means the solvent strength must be increased.
All color surfaces covered by overpaint or polymerized synthetic resins are in danger of dissolving, causing pigments with lesser covering and bonding properties to abrade. This is true of all carbon and organic colors, such as black, umber, earth green and earth red, and the like.
Typically when a solvent is applied to a polymerized film, the film resists dissolving. As the solvent continues to penetrate the film, soluble material locked in the paint film, such as triglycerides of the saturated kind formed from palmitic and stearic acids, starts to diffuse out of the film. This leaching causes a swelled state as the volume of the film increases and softens. The longer the paint film is exposed to the chemical action of a solvent, the greater the leaching action. The paint film may not dissolve after leaching and swelling has taken place but it is more brittle and has a decreased volume due to leaching. The chemical and physical bonding state of such paint films are permanently weakened and will most likely be susceptible to abrasions and paint loss in the future when the varnish layer used to replace the one cleaned must itself be removed. All Old Master paintings that have been cleaned in this century will be susceptible to this inevitable damage when they are next in need of cleaning.
The chemical and physical problems which endanger paint films when natural resin and synthetic varnishes are removed by aromatic solvents has been defined so that conservators have been aware of the problems which surround the cleaning of a painted surface. All varnish layers discolor and yellow over time, and, in so doing, obscure the color surface. This problem is compounded when the original color surface has been overprinted or covered with mediums which polymerize or partially polymerize with the color surface.
Lasers can be used to safely remove varnish and overpaint adhesives which cover a paint surface without endangering the oil paint film. This requires that the power, exposure time, pulse width, frequency, or wavelength have been properly selected and adjusted to the paint surface encrustation. In cases of overpaint, the laser's ability to vaporize inorganic bonds has been technically possible for many years. In the case of organic varnish layers, the removal has been technically possible for a number of years.
Objections to the use of lasers have been centered around the cumulative thermal effects of the exposure on the organic paint film. This objection is valid when purely heating effects are used to remove layers, and successful tests to remove overpaint and cross-linked synthetic varnish were conducted with argon and ruby lasers as early as 1978.
The present invention uses infrared wavelengths, such as a pulsed Erbium:YAG laser, and has shown that application of a pulsed laser with selected thermal interaction can successfully remove polymerized overpaint and synthetic varnishes as well as organic encrustation, adhesives, and natural resin varnish without endangering the color surface. This is possible because of the selection of a laser wavelength and power which allows minimal penetration and selective non-thermal removal of organic materials. Films of opaque or semi-opaque characteristics are safely removed by vaporizing the adhesive bonding without penetrating or heating the original color film surface. This control is absolutely impossible when considering the penetration of aromatic and alkaline solvents. This is true even when the solvents are suspended in gel because the penetration of the solvent gas which dissolves the varnish resin or overpaint creates the same internal swelling.
Prior U.S. patents which use lasers in cleaning surfaces can be seen in the Boquillon et al. U.S. Pat. No. 5,151,134, for a method and device for cleaning a surface with a laser. A process and device for cleaning pollutants from a surface uses a laser for applying laser pulses to the surface to be cleaned in which the laser is pulsed in pulse durations not exceeding 30 nanoseconds. The area on the surface is contacted with the pulsating laser and the surface is cleaned in the absence of an observably thermal effect and the laser is removed from the area on the surface after the area is cleaned. The process removes pollutants from a surface of material selected from the group consisting of stone, glass, steel, ceramics, wood, paper and cardboard. The Lovoi et al. U.S. Pat. No. 4,588,885, is a method of an apparatus for the removal of paint and the like from a substrate. A method and apparatus controls the stripping of paint from a substrate by pulses of high intensity radiant energy. The Woodroffe U.S. Pat. No. 4,756,765, uses a laser to remove poor thermally conductive materials, such as paint, grease, and ceramics, from a substrate by ablation without damaging the substrate by delivering to the material to be removed pulses of a laser beam having a wavelength at which the material to be removed is opaque. Laser energy is used which is sufficient to ablate or decompose the material without damaging or adversely affecting the substrate or its surface. The Kumar U.S. Pat. No. 5,268,548, is a method of removing paint and other coating from large and small substrate structures including applying to the surface of the structure a compound capable of coupling with a microwave radiation in the wavelength range for causing pyrolysis of the paint.
The following articles also deal with the cleaning of surfaces using lasers. In Applied Optics, Volume 34, No. 21, Jul. 20, 1995, an article by Katherine Liu and Elsa Garmire discusses paint removal using lasers as a practical way to remove graffiti from building walls. A variety of lasers were tested and the article suggests a Q-switched Nd:YAG laser as the most efficient means for removing graffiti and unwanted paint. An article entitled "Lasers for Art's Sake|" in Optics & Photonics News of May, 1995, by Costas Fotakis discusses using modern laser technology to improve the conservation and non-destructive diagnostics and composition analysis of paintings. In particular, Excimer lasers are indicated as a promising tool for surface cleaning of paintings, cleaning support materials, such as a canvas, paper, or wood, and recovering original paintings from over-paintings. The technique is based on the controllable removal of surface layers by photoablation. In the NASA Tech Briefs of April, 1996, entitled "Atomic Oxygen Removes Varnish and Lacquer From Old Paintings" a dry relatively nondestructive plasma process is suggested to remove protective coats of varnish and/or lacquer from old paintings. In an article in Optics & Laser Technology, Vol 27, No. 1, 1995 by M. I. Cooper, D. C. Emmony, and J. Larson entitled "Characterization of laser cleaning of limestone", a precise cleaning of a polluted limestone sculpture by a Q-switched Nd:YAG laser is described. In the Electronic Engineering Times of Monday, Aug. 14, 1995, Issue 861, an article discusses using a laser in cleaning articles and supports using an Excimer laser to clean roman coins and metal plates. These prior articles indicate an interest in the use of lasers for cleaning various types of artworks including oil paintings.
The present invention is a process for improving the cleaning of a painted surface using a laser in the removal of varnish films, polymerized over-paint, adhesives, calcium carbonate gesso layers, and the like, which cover and obscure paint, lacquered or patina films on canvas, wood, cardboard, paper, leather, parchment, metals, terracotta, marble and stone supports. Improvements result from the selection of a laser with a predetermined wavelength and power to allow minimal penetration used in combination with the presence of an OH radical in the substrate to be removed or which is first applied to an area of the surface prior to the application of laser energy to the coated painted, lacquered or patina surface and, in some cases, the use of a cleaning agent to remove the loosened materials from the laser treated coated area.